This invention generally relates to a nonwoven fabric for use in filtration of food and non-food liquids and semi-solids, particularly dairy products such as milk, yogurt and cheese. The fibers in the nonwoven fabric of the invention are primarily thermally bonded.
Nonwoven fabrics for use as filters are known in the art. Typically, filter disks, socks or tubes are formed from a binder-impregnated sheet of nonwoven fibers. The filter sheet is fabricated to have a porosity for filtering sediment and other contaminants from a liquid or semi-solid, such as raw milk from a dairy farm.
The most critical physical milk filter properties are:
1xe2x80x94high flow rate; 2xe2x80x94good sediment retention; 3xe2x80x94high wet burst strength; 4xe2x80x94high wet tensile strength; 5xe2x80x94good surface wettability (i.e. the substrate is very hydrophilic). The surface of the web must have good fiber and wood pulp tie down in which most of the fibers are bonded well to the surface of the substrate.
Milk filters having a minimum wet burst strength of about 40 to about 50 psi are known to perform well in the field. High flow rate signifies the ability to filter a greater volume of milk before the filter becomes plugged with undesirable sediment. High flow rate filters are particularly advantageous in light of the consolidation of small farms into large dairies. Filters used in connection with comestibles must also meet all relevant government regulations. For example, the current U.S. Food and Drug Administration regulations for milk filters require that the extractable mass must be below 2.8% of the dry weight of the test sample after two hours in distilled water at 100xc2x0 F. (37.78xc2x0 C.).
One example of a prior art filter, U.S. Pat. No. 4,973,382, describes a wet laid fabric made from fibers of dissimilar denier and length which are secured together with a liquid binder in an amount of 20-35% based on dry fabric weight. U.S. Pat. No. 4,973,382 is incorporated in the present application by reference. Using a large amount of liquid binder in a milk filter has the disadvantage that large bond points are formed at the fiber interstices, which can significantly reduce the fabric""s open pore area and adversely affect flow rate.
The filtration fabrics of this invention have an improved flow rate averaging no more than 25 seconds, preferably averaging no more than 21 seconds, while maintaining the prior art""s average sediment retention of 70% and minimum wet burst strength of about 40 psi to about 50 psi, preferably about 50 psi. Some of the filtration fabrics demonstrate improved flow and increased filtration properties such as sediment retention. A particular preferred embodiment of the invention results in a product having a sediment retention of at least 79% and a flow rate of about 11-12 seconds.
The filtration fabrics can be produced having the following composition:
Various furnishes known within the art can be employed as needed. A furnish is defined as all of the fibers and pulps combined to make the nonwoven product.
The furnish fiber percentages and viscosity modifier amounts are calculated as a percentage of the dry weight of the furnish components. The amount of rewetting or nonrewetting surfactant added in the furnish is also calculated as a percentage of the dry weight of the furnish components.
The amount of latex binder or other stiffening agent is calculated as a percentage of the total dry weight of the final fabric. In a preferred embodiment of the invention, the percentage of latex binder is about 4 to 5%. The amount of rewetting surfactant added with the binder is calculated as a percentage of the total dry weight of the final fabric.
The basis weight can vary from 0.5 oz/yd2 to 5.0 oz/yd2, preferably 1.5 oz/yd2 to 2.5 oz/yd2.
Binder fibers can include the following bicomponents: co-polypropylene, polypropylene/polyethylene, polyester/polyethylene, co-polyester, and co-nylons. Side-by-side or sheath/core bicomponent binder fibers can be used. Sheath/core, bicomponent fibers are the preferred binder fiber and can be concentric or non-concentric. These preferred fibers are characterized in that the sheath portion of the bicomponent has a melting temperature lower than the core portion of the fiber. The melting temperature of the sheath portion can be about the same or lower than the temperature at which the newly formed wet laid sheet is dried, and the melting temperature of the core can be about the same or above the drying temperature. In the preferred embodiment, the sheath has a melting temperature lower than the core and slightly lower than the temperature at which the fabric is dried. Polyvinyl alcohol or other monocomponent binder fibers can be utilized.
Other co-binders, which are suitable, as filter media for liquefied food and beverages include polyvinyl acetate, ethylene vinyl acetate, methyl and/or ethyl acrylate homopolymers and copolymers, acrylic/styrene copolymers, and styrene-butadiene copolymers.
Synthetic matrix fibers such as acrylics, nylons, polyesters (e.g. polyethylene terephthalate (PET)), polypropylene, and polyvinyl alcohol, wood pulps, natural fibers such as rayon, cellulose acetate and cotton can be included, individually or in various combinations, as the pulp portion of the filters of this invention.
Various rewetting and non-rewetting surfactants, viscosity modifiers and stiffening agents known to the art can also be employed as desired for a particular application. A rewetting surfactant promotes rewetting once fabric sheets are dried, whereas a non-rewetting surfactant will not promote rewetting once fabric sheets are dried. A preferred rewetting surfactant is sodium dioctyl sulfosuccinate. It is well known in the art to select among commercially available products to match process capabilities.
Other objects, features, and advantages of the present invention will be explained in the following detailed description of the drawings having reference to the appended drawings.